1. Field of the Invention
The invention relates to a vehicle running control apparatus and method which includes wheel speed detection means for detecting a wheel speed based on a pulse signal corresponding to a rotational speed of a wheel, and which controls a vehicle running state according to a vehicle running situation that is determined based on the wheel speed, and a technology for determining the vehicle running situation.
2. Description of the Related Art
A vehicle is known, in which a vehicle running situation is determined using, for example, an acceleration of a wheel that is calculated from a wheel speed based on a pulse signal corresponding to a rotational speed of the wheel. In the vehicle, it is possible to determine that the vehicle is running on a rough road having a high running resistance as compared to a normal flat paved road, for example, the vehicle is running on a sand road, or that the vehicle is running while towing something. An example of the vehicle is disclosed in Japanese Patent Laid-Open Publication No. 6-48202 (JP-A-6-48202) or Japanese patent Laid-Open Publication No. 9-48254 (JP-A-9-48254). The vehicle disclosed in the aforementioned publication includes a frictional engagement device for distributing driving torque generated by a driving force source to part of plural wheels, such as an electromagnetic clutch device or a hydraulic clutch device that is provided in series with a propeller shaft in order to select between a four-wheel-drive state and a two-wheel-drive state, or to control a ratio at which power is distributed to a front wheel and a rear wheel in the four-wheel-drive state. In the vehicle, the vehicle running situation is determined based on at least the acceleration of the wheel, and engagement force of the frictional engagement device is controlled according to the result of the determination, whereby the driving force is appropriately distributed to the front wheels and the rear wheels, and stable running through performance can be obtained.
It is considered that the acceleration of the wheel is generally obtained by differentiating the wheel speed. However, in the case where the wheel speed is detected based on the frequency of the pulse signal, the frequency may become equal to or lower than a certain reference, and the number of the pulse signals during a certain time may become zero in an extremely low speed region, for example, in a region where the wheel speed is approximately 3 km/h or lower. Therefore, the wheel speed may be detected as zero in the extremely low speed region. In this case, when the wheel speed reaches a predetermined wheel speed, the acceleration of the wheel that is a differential value of the wheel speed sharply increases. Thus, since the accuracy of determining the acceleration of the wheel is low, there is a possibility that the vehicle running situation cannot be appropriately determined. Also, there is a case where a determination on the vehicle running situation is prohibited in the extremely low speed region since the accuracy of determining the acceleration of the vehicle is low in the region. In this case, there is the following problem. While the vehicle is on a road having a high running resistance, it is desired to bring the vehicle into the four-wheel-drive state suitable for running on a road having a high running resistance, for example, in order to move out of a sand road when the wheels start moving, that is, when the vehicle speed is zero, or to run on the sand road. However, since the vehicle running situation is not appropriately determined in the extremely low speed region as described above, there is a possibility that the driving force is not distributed to the front wheels and the rear wheels as desired, and therefore stable running through performance cannot be obtained.